Foreach, `List`

With foreach we can loop over the items in a List. This works just like the for-loop, but with simpler syntax. But there is a limitation.

An exception

We cannot modify a List while we use foreach on it. A "Collection was modified" exception will be thrown. Foreach requires the collection not change under it.

Required output

Consider a list that has 3 elements. In our foreach-loop, we want to safely iterate over the 3 elements in order.

10, 100, -1

Iterations: 10
            100
            -1

Foreach example

This example has 2 parts. It shows an exception that can occur when using for each on a List. In our code, it is best to avoid this exception.

Part 1 In the first foreach loop, we correctly loop over the ints in the List.

Part 2 In the second foreach loop, we create an error by trying to add an item to the List while in a foreach.

Important We must not modify a List inside a foreach-loop on that List. Other Lists can still be modified though.

using System;
using System.Collections.Generic;

List<int> list = new List<int>();
list.Add(10);
list.Add(100);
list.Add(-1);

// Part 1: we can loop over list items with foreach.
foreach (int value in list)
{
    Console.WriteLine(value);
}
Console.WriteLine("::DONE WITH PART 1::");

// Part 2: this will cause an exception.
foreach (int value in list)
{
    list.Add(0);
}10
100
-1
::DONE WITH PART 1::

Unhandled Exception: System.InvalidOperationException:
Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper .ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections. Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections. Generic.List`1.Enumerator.MoveNext()
   at Program.Main() in...

A solution

How can we modify a List while looping over it? There is a solution: we can use a for-loop. The for-loop has no "Collection was modified" limitation.

A summary